Dr Peter Landschützer, Dr Katharina Six and Dr Irene Stemmler, scientists at the Max Planck Institute for Meteorology (MPI-M), and co-authors were able to quantify the increase in the seasonal amplitude of the annual marine CO2 cycle for the first time on the global scale. Their findings have recently been published in Nature Climate Change.

The increase in atmospheric CO2 has multiple consequences for the ocean carbon cycle and marine life. The best known is ocean acidification resulting from the uptake of this excess CO2 by the ocean and the subsequent chemical reactions that acidify the ocean. A much less well known, but potentially also important consequence is the increase in the amplitude of the seasonal cycle of CO2 in the ocean. Thus far though, the observational evidence for this increase has been lacking on a global scale. Now, an international team of researchers led by MPI-M together with scientists from the Swiss Federal Institute of Technology Zurich (ETH Zurich) and the University of East Anglia have verified this prediction.

Using the to date largest collection of surface ocean CO2 measurements in combination with a neural network-based data interpolation technique, Landschützer and co-authors were now able to quantify this increase in amplitude for the global ocean surface waters from 1982 through 2015. Their results suggest that the added CO2 has led to an increasing seasonal cycle of 2.2±0.4 µatm/decade, which is in broad agreement with independent measurements from time series stations and basic thermodynamic considerations. While the study highlights the advances of the surface ocean observing networks in our ability to validate model predictions, the study also emphasizes the detectable imprint human emissions have already left on the global oceans over the past three decades. The results of the study are therefore far from good news as the increasing seasonal variation amplifies the occurrence of ocean acidification by inducing the transition through critical thresholds that are harmful to marine organisms earlier in time.

Dr Peter Landschützer, lead-author of the study, says: "The increasing seasonality should not come as a surprise, since it has been predicted by several studies before. What our study tells us, however, is that the observational network in place can be used to verify model predictions on the global scale, which is of increasing importance in light of the Paris climate agreement. In addition, the study shows that climate change is not worries for later and the increasing CO2 seasonality in the global oceans is one more piece of evidence that we humans have already left a detectable imprint on our planet."

Dr Dorothee C. E. Bakker, researcher from the University of East Anglia adds: "The study highlights the importance of the ongoing data collection and synthesis efforts by seagoing scientists for detecting the pervasive influence of human activity on the oceans. Annual public releases of the Surface Ocean CO2 Atlas (www.socat.info), which this study has used, are a Voluntary Commitment to the 2017 UN Ocean Conference (#OceanAction20464)."

Prof Nicolas Gruber, Professor at ETH Zurich, Switzerland emphasizes that "Our work shows again that the large service to humanity provided by the ocean thanks to it taking up about 30 percent of the global human-made emissions has negative consequences for marine life. To understand why an increase in the seasonality of CO2 could be detrimental for marine organisms, one can take the seasonal cycle of temperature as an example. Our detected increase in the seasonality of CO2 is analogous to winters getting colder, and summers warmer. Thus, if an organism is sensitive to e.g., hot conditions, a trend towards warmer summer will greatly enhance its exposure to potentially deleterious conditions, e.g. induced by a heat wave."